Add configurable mode indicator for the vi editing style

[h-3-0]

Fixes #38.

When using the vi editing style, the presence of insert-mode or command-mode is indicated before the usual prefix by a user configurable string.

The emacs editing style, and all other modes (e.g. search-mode, arg-mode) are unaffected.

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This change is 

[CarlosMChica]

@judah I'd be interested in getting this feature in, what's the reason for not merging this PR?

Thanks in advance.

[judah]

I am in favor of the feature. However, I feel that this particular PR is too invasive compared to what it is implementing. It involves adding a new parameter to the low-level core of the rendering loop, for something that is only specific to display of the vi mode.

Instead, I would prefer the changes be more localized to the Vi.hs module. One half-formed idea is that it ought to be possible to write functions like:

changeInsert :: ReaderT Prefs m => (s -> IMode) -> Command m s IMode
changeInsert f s = do
    indicator <- lift asks insertModePrefix
    let t = f s
    setState (Prefixed indicator t)
    return s

data Prefixed s = Prefixed String s
instance LineChars s => LineChars (Prefixed s) where ...

Which would render the indicator prefix when the state is changed.

Another idea is to actually keep the Prefixed s itself as the line state inside the Vi loop, using its Functor instance to turn functions like IMode -> IMode into Prefixed IMode -> Prefixed IMode. I'm not sure which way would be simpler, though.